1. Field of the Invention
The present invention relates to a lens-interchangeable digital camera system and a digital camera, interchangeable lens and camera body thereof.
2. Related Art Statement
A conventional digital camera includes an optical low-pass filter (called optical LPF hereinafter), containing crystal or the like having a double refraction characteristic, in the front of the surface of an image pickup element in order to reduce false color (such as moiré) of a high frequency component of a shot image. A single lens reflex digital camera system has been provided in which a lens (lens barrel) is interchangeable with respect to the camera body. Image pickup elements having different numbers of pixels, that is, having different pixel pitches are built in multiple bodies of the lens-interchangeable digital camera system. Furthermore, multiple kinds of interchangeable lens can be attached to the body.
An optical LPF having thickness corresponding to the pixel pitches is provided in the front of the surface of the image pickup element having different pixel pitches as described above in the single lens reflex camera body.
The thickness of the optical LPF is set in accordance with the pixel pitch of the image pickup element as described above because a luminous flux having passed through the optical LPF is divided into non-refractive normal light and abnormal light (double refractive) and the divided luminous fluxes must be launched into adjacent pixels of the image pickup element in order to prevent the occurrence of the moiré. Therefore, the thickness of the optical LPF must be changed in accordance with the pixel pitch.
For example, Japanese Patent Application Publication No. Hei 7-123421 and No. 2000-244821 and Japanese Patent Publication No. 2552855, which will be described later as Patent Document 1, Patent Document 2 and Patent Document 3, respectively, discloses propositions relating to optical path length control and/or spatial frequency characteristic control by operating an optical LPF in a conventional digital camera.
An image pickup apparatus disclosed in Patent Document 1 has a switching circuit for switching a first image pickup mode and a second image pickup mode. In the first image pickup mode, color natural image signals are generated by using electric signals output from an image pickup element. In the second image pickup mode, high resolution image signals are generated for a single colored or monotonous image. An optical LPF is retractable in accordance with the switching, and the change in optical path length is corrected by using another optical system. With the image pickup apparatus, high resolution image signals can be generated for a monotonous image.
An image pickup apparatus disclosed in Patent Document 2 drives switching of a dummy glass for optical length correction and an optical LPF corresponding to driving modes of an image pickup element. The dummy glass for optical path length correction and the optical LPF constitute an LPF block. With the image pickup apparatus, the occurrence of pseudo signals caused by changes in spatial sampling characteristic due to special driving of the image pickup element can be effectively suppressed.
In an image pickup apparatus disclosed in Patent Document 3, when a frame reading mode is selected, correction optical means, instead of an optical low pass filter, is inserted to an incident optical path onto an image pickup means. The correction optical means has the same optical length as that of the optical LPF. With the image pickup apparatus, aliasing does not occur even when the vertical scan frequency for sampling is changed from 525 to 262.5. Therefore, the deterioration in image quality does not occur.
Some single lens reflex digital camera system may apply LN element (LiNbO3) as an optical LPF, instead of crystal. The LN element is extremely thin and has the same double refraction characteristic as that of crystal. In order not to significantly change pixel pitches in every camera body of the digital camera system, the size of the image pickup element may be changed so as to change the number of pixels. When the LN element is applied, the optical path length of the luminous flux does not significantly change since the LN element is thin. Therefore, an interchangeable lens can be interchanged between digital camera bodies having different pixel pitches.